Prostate adenocarcinoma is the most commonly diagnosed cancer in the U.S. male population (excluding skin cancer). Over 20% of these cases are locally-advanced non-metastatic cancers. Treatment for this stage is problematic with significantly low control rates using traditional doses of radiation, which is the main-line therapy. Treatment of prostate cancer is difficult because of the extreme proximal position of tissues that are sensitive to radiation, such as the bladder and rectum. Radiation treatment, which is typically delivered in daily fractionated doses over the course of several weeks, is further complicated by prostate motion relative to the radiation field on a daily basis. More aggressive radiation treatment techniques utilizing conformal fields and higher doses have been used with improved therapeutic results. However, these dose-escalated treatments have met with problems due to increased dose delivered to normal tissues that are in the radiation field, producing many unacceptable complications such as rectal fistulas and bladder perforation and/or sloughing. Therefore, dose-escalated, conformal treatments cannot be delivered without significantly increased morbidity unless the exact position of the prostate can be visualized and registered, and this field localization maintained during the course of the treatment.
The following sections describe in more detail the current treatment model for external beam radiation therapy, including the equipment involved, the procedural methods or phases involved, and the existing problems and limitations.
A linear accelerator (“LINAC”) is a treatment device which generates a beam of therapeutic high-energy X rays or electrons. The treatment focus of the beam is the isocenter, which exists at a fixed location with respect to a movable gantry. Moving the gantry allows the angular orientation of the beam (but not the location of the isocenter) to be adjusted. A movable treatment table allows the position and orientation of the isocenter to be adjusted with respect to the patient. The cross-sectional size and shape of the beam can be modified by adjusting the rectangular aperture and by obscuring portions of the resulting rectangular beam, using either custom-cut lead blocks or an automatic multileaf beam collimator. The position of the isocenter for a specific LINAC installation is indicated by orthogonal laser beams. This positional information aids the treatment technician in positioning the patient, treatment table and gantry correctly prior to each treatment. The lasers are aligned with ink marks made on the patient's skin.
An X-ray simulator is a treatment planning device which uses low-energy diagnostic X-rays to simulate an external-beam LINAC treatment. The simulator is a low-energy X-ray unit with a movable gantry and treatment table similar to that of the LINAC. Low-energy beams are directed through the patient at the same angles of incidence which will be used during treatment. The resulting “beams-eye” X-ray images are captured on film and imported into a treatment planning system, where beams are defined, sized, and blocked, and the resulting dose distribution is predicted.
A CT simulator is a treatment planning device which captures transverse CT images referenced to a simulated isocenter. The resulting CT view volume is typically imported directly into a treatment planning system, where beams are defined, sized and blocked, and the resulting dose distribution is predicted. The CT Simulator provides more information than the X-Ray Simulator, because additional anatomical information, including the density of intervening tissue, is visible.
Treatment planning systems include third-party software applications that enable an operator to graphically specify beam apertures conformal to the prostate, based on externally-obtained image data. The radiotherapeutic dose resulting from the specified beams is then computed, and decisions are made with respect to beam energy, number of planned treatments, etc.
The first step in radiation treatment involves simulation, during which an X-ray simulator or CT simulator is used to capture anatomical information about the patient referenced to a simulated treatment isocenter. Using indelible ink, marks are made on the patient's skin indicating the location of the simulated isocenter. Later, these marks will be used to align the patient during treatment. The input to this process is the number of beams to be used and the angles of incidence of each beam, which correspond to the positions of the LINAC gantry to be used at treatment time. Typically, four or six beams are defined. The output of this process is either X-ray images or a CT view volume, spatially referenced to the simulated isocenter.
The second phase involves treatment planning, during which a radiation physicist and radiation oncologist design a multi-beam treatment plan for the patient using a Treatment Planning System (TPS). The input to this process consists of the isocenter-referenced X-ray images or CT view volume resulting from the simulation process, as well as information on the specific LINAC system to be used during treatment. A urologist or radiation oncologist determines the presumed location of the prostate with respect to the isocenter and “contours” or delineates its outline to the TPS. The oncologist defines the apertures and blocking for each beam, thereby defining the cross sectional geometry of each beam. Beams are defined so that the volumetric intersection of the beams conforms as nearly as possible to the presumed location and extent of the prostate. The output of this process is configuration information for the LINAC, including beam apertures, block geometry, beam energy, and beam orientation and also treatment methodology, including the number and frequency of treatments.
The third stage of the LINAC process is the actual treatment delivery, during which a radiologist aligns the patient with respect to the isocenter, using the guidance lasers associated with the LINAC and the ink marks made on the patient's skin during simulation. This is accomplished by moving the patient and/or treatment table as necessary. For each beam defined in the treatment plan, the LINAC is set up with the appropriate gantry angle and beam configuration (field size and blocking), and the specified radiation dosage is delivered.
One of the primary problems associated with radiation treatment of prostate cancer is the location of the prostate during treatment planning. The prostate is not visible on simulation X-rays and is difficult to define in simulation CT data. As a result, during treatment planning, the oncologist must make a judgment determination as to the location of the prostate by reference to nearby structures (e.g. pelvic girdle, bladder, etc.) Variations between patients, especially in prostate size, make this an imperfect process. The resulting beam definitions are not optimally conformal with respect to the prostate, resulting in potential under-dosage of the cancerous tissue and/or overdosage of nearby healthy tissue. The ability to accurately determine the location and extent of the prostate during the treatment planning process would result in better beam/prostate conformance and allow more accurate treatment delivery.
Another significant problem during radiation therapy is caused by prostatic movement between treatment sessions. The patient is positioned at treatment time by aligning the LINAC guiding lasers (indicating the position of the isocenter) with the ink marks on the patient's skin indicating the location of the simulated isocenter. Normal migration of the prostate within the body due to bladder contents, rectal contents, prostatic edema, hormonal therapy, and other factors cannot be accounted for at treatment time. Since numerous treatments are delivered over a period of weeks or months, this migration can result in significant loss of targeting accuracy due to prostatic movement with respect to the isocenter.
Likewise, there is also an issue of prostatic movement during the actual treatment session. After the patient is positioned for treatment, the operator leaves the room and administers the treatment remotely, typically viewing the patient via a closed-circuit video link. Any movement by the patient may move the prostate with respect to the treatment isocenter, reducing beam/prostate conformance and impairing the effectiveness of the treatment.
Another significant issue is the unwanted radiation exposure to the rectum and bladder. Due to the proximity of the rectum to the prostate, treatment plans must be careful to avoid overdosing the rectal wall and the bladder in the course of treating the prostate. The amount of fecal matter in the rectum and the volume of bladder content can affect the dosage received by the posterior wall of the rectum or by the bladder during any given treatment.
One conventional system is marketed as the BAT (B-mode Acquisition Targeting) system by Nomos Corporation. The BAT consists of a transabdominal ultrasound probe attached to a table-mounted localizer arm, and a two-dimensional ultrasound imaging system, which is used to display the prostate during the process of positioning a patient with respect to the isocenter at treatment time. BAT does not offer a treatment planning component.
The BAT system uses a transabdominal TA probe, which can be used by a radiation technician with minimal increase in treatment time, instead of a transrectal (TR) probe. The TR probe provides more reliable imaging of the prostate than the TA probe, since the amount of intervening tissue between the rectum and prostate is small and patient size has little effect on the relevant geometry. Patient size can have a significant effect on the ability of a TA probe to view the prostate.
The BAT provides two-dimensional imaging and must be moved by an operator to offer different spatial views of the prostate. The BAT cannot be used during treatment, because it would interfere with the therapeutic beams and because it would be difficult to ensure continued ultrasound-transparent contact with the patient throughout the treatment. Consequently the BAT is used only during patient set-up. Furthermore, the BAT is not integrated with the treatment plan and is only used to visually position the center of the prostate with respect to the isocenter.